I'll reserve judgement on whether or not the mask could increase VO2 max.
As for the ankle weights: ankle weights could actually cause injury due to an excessive load capacity on smaller muscles in the legs.
I teach physiology to medical students. For the reason I mentioned above, I very much doubt this mask is going to increase VO2max. Unless you wear it all the time, you really aren't going to get an increased in number of red blood cells or hemoglobin content, which is the underlying mechanism. Kidneys take some time to respond with increased in erythropoietin production. It would take more than one very well designed study to convince me that these things work that way. Another possibility is that they increase diaphragm and intercostal muscle strength. But again, I wonder if that can really happen much.
But I want to echo strongly the comments about the leg weights and effects on ankles and hip. Trust me, the last thing you want is to induce damage to Achilles tendon if you are free of any problems there at the moment, so if you use the wieights do it with caution and not for very long at a time. (I have seen Chinese coaches here do some things to students that are almost guaranteed to cause injuries, which is the way they were trained, so I would say that not
all of their training methods are equally well informed in terms of modern sports medicine and exercise science).
A valsalva maneuver may be part of the reason some people hold their breath, but I doubt it is the main reason in TT. I think psychological reasons related to stress are a better explanation most of the time -- in this sport anyway. The reason I think that is people will sometimes hold breath almost throughout the rally, or at very lest over a period of time that covers several shots. You really do need to sometimes remember to breathe, and again, the grunting or vocalization during a shot is one way to make sure it happens. It also helps with timing. It is also really annoying to everybody else who has to listen to it. Always a downside.